7/30/2014 3:45 AM PDT BY TMZ STAFF

Breaking News

Keith Urban's alcohol-crazed show Saturday outside Boston created more than a drunken mob scene ... police say it may have fueled a rape ... right there at the venue.

18-year-old Sean Murphy has been arrested for allegedly sexually assaulting the 17-year-old girl ... right in front of several witnesses who stood by and filmed it with their cell phones but did nothing to thwart Murphy.

Cops say both Murphy and the girl had consumed alcohol.

Murphy pled not guilty, claiming the intercourse was consensual. FYI ... the age of consent in Massachusetts is 16.

Murphy's attorney, Neil Crowley, tells TMZ, "This was a consensual act, not a sexual assault. There are no allegations of force or violence against him. This was a private act that regrettably occurred in a public place. The young woman was neither intoxicated nor overcome by drugs at the time."